Memcached sharding is sharding, not replication. That is, there isn't a copy of the data on each server; the data is split up between servers.
Keys will only ever exist on one server. Dalli passes keys through a distribution algorithm to determine which server will own a given key, then passes messages to that server. The same keys will not exist on multiple servers, so you don't have to worry about expiring the keys on both of them at once. If you expire the key, it will be expired on the single box that it lives on.
If both users are requesting data via the same cache key, then expiring that cache key will invalidate it for both of them; you don't have to worry about User 1 getting routed to Box 1, and User 2 getting routed to Box 2, because a given key will always map to a given server.